Portable universal friction testing machine and method

ABSTRACT

A friction testing machine and method for measuring friction characteristics between a test sample and a friction surface. The machine and method are particularly suited for measuring the coefficient of friction between a rubber specimen or a tread element and different friction surfaces at different sliding velocities, contact pressures and orientations. The machine includes a carriage, a friction surface, a motion device, a sample holder, a variable weight loading device, and a force measurement device. The motion device to cause relative movement between the carriage and the friction surface in forward and reverse directions and the sample holder holds a sample in frictional engagement with the friction surface during this forward and reverse movement. The variable weight loading device loads the sample holder so that a selected load can be applied to the sample in a direction normal to the friction surface. The force measurement device obtains a measurement indicative of the frictional force resisting movement of the sample as it is moved in the forward and reverse directions. The processor controls the motion device, controls the variable weight loading device and/or records the measurements obtained by the force measurement device.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/276,898 filed on Mar. 25, 1999 now U.S. Pat. No. 6,199,424,which is a divisional of U.S. patent application Ser. No. 08/835,340filed Apr. 7, 1997 now U.S. Pat. No. 5,900,531. The entire disclosuresof these earlier applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention herein described relates generally to friction testmachines and methods and, more particularly, to a friction testingmachine and method for measuring the coefficient of friction between arubber specimen or a tread element and different friction surfaces atdifferent speeds, contact pressures and orientations.

BACKGROUND OF THE INVENTION

When developing new compounds for tires, a prior practice was to build atire and subject the tire to various traction tests. Because of the costand time needed to build test tires for the purpose of optimizingtraction properties, small samples of tread compounds have instead beenused to determine the traction characteristics of the compounds. Byusing small samples for testing, a large number of different treadcompounds can be made in small batches for screening to determine whichcompound has the best properties. The small samples could also be testedwith different friction test surfaces representing different roadsurfaces and different conditions, whereby the traction properties ofthe compounds may be observed to determine which compound has the besttraction on a specific road surface under specific conditions.

In U.S. Pat. No. 5,113,688 there is disclosed an apparatus and methodfor predicting tire traction characteristics of tread compounds usingsmall test samples. The apparatus causes a rotating relationship betweenthe sample and friction surface. The torque between the sample andfriction surface is measured and a torque versus time curve isestablished for the sample. Such apparatus, however, does not appear tobe easily movable as may be desired for testing at different locations,nor is such apparatus suitable for evaluating the performance ofdifferent tread patterns. It would be advantageous to have a portablefriction testing apparatus that not only can measure the coefficient offriction of plain tread elements, but also can measure the coefficientof friction of tread blocks, shoulders, etc., of existing tires. Afurther desirable advantage would be an apparatus that permits one toview the interaction between the friction surface and the specimen inthe same way as the present invention.

SUMMARY OF THE INVENTION

The present invention provides a friction testing machine and method formeasuring friction characteristics between a test sample and a frictionsurface. The machine and method are particularly suited for measuringthe coefficient of friction between a rubber specimen or a tread elementand different friction surfaces at different sliding velocities, contactpressures and orientations. A preferred embodiment of machine isself-contained and portable, configured for easy and quick changing ofthe friction surface, and provides for rotating the test sample about anaxis normal to the sliding surface and the direction of movement of thesample relative to the friction surface.

In general, a friction test machine according to the invention comprisesa sample holder configured to hold a sample in frictional engagementwith a friction surface, and a motion device for effecting relativemovement between the friction surface and sample holder in a firstdirection. Also provided is a variable weight loading device on thesample holder for loading the sample holder such that a selected loadcan be applied to the sample normal to the friction surface, and a forcemeasurement device for obtaining a measurement indicative of thefrictional force resisting such relative movement between the sampleholder and the friction surface effected by the motion device.

Preferably, there is provided a second motion device for effectingrelative movement between the sample holder and friction surface inanother direction different than the first direction, such as rotationabout an axis perpendicular to the friction surface and/or translationin a direction transverse to the primary direction of movement.

In one embodiment of friction test machine according to the invention,there is provided a base on which the friction surface is supported, anda carriage mounted on the base and guided for movement in a firstdirection parallel to the friction surface. The motion device isconnected between the carriage and base for moving the carriage in thefirst direction, and the sample holder is mounted to the carriage formovement therewith and configured to hold the sample in frictionalengagement with the friction surface during movement of the carriage inthe first direction. In another embodiment of the friction test machine,the motion device causes relative movement between the carriage and thefriction surface in a second direction opposite to the first directionso that the force measurement device can obtain measurements indicativeof the frictional force in forward and reverse directions.

Preferably, the friction surface is provided on a friction element, andthe friction element is removably supported on the base, whereby thefriction element can be interchanged with other friction elements forproviding respective different friction surfaces. In a preferredembodiment, the base includes a recess for removably holding thefriction element, and the base is supported on a cart for easy transportof the machine from place to place.

In another embodiment of friction testing machine according to theinvention, there is provided a carriage fixed to the base, and thesample holder is mounted in the carriage for vertical movementperpendicular to the friction surface. A table is mounted to the basefor rotation about a vertical axis, and the friction surface extendsannularly around a radially outer peripheral portion of the table,whereby the friction surface is moved while the sample remainsstationary.

In still another embodiment of friction testing machine according to theinvention, there is provided a base adapted to rest on a surface againstwhich a sample is to be tested, such as a road surface. A carriage ismounted on the base and guided for movement in a first directionparallel to the friction surface. A motion device is connected betweenthe carriage and base for moving the carriage in the first direction,and a sample holder is mounted to the carriage for movement therewithand configured to hold the sample in frictional engagement with thefriction surface during movement of the carriage in the first direction.Preferably, the base is provided with an aperture through which thesample holder extends for positioning the sample against the frictionsurface disposed beneath the base.

In a preferred embodiment of the invention, the variable weight loadingdevice includes a weight platform on which removable weights can bestacked and removed to selectively vary the load applied to the sample.Alternatively, the variable weight loading device comprises a fluidcylinder that is controlled by the processor to vary the load applied tothe sample. It also is preferred to provide the friction surface on afriction element that is easily and quickly interchangeable with otherfriction elements for providing respective different friction surfaces.

In a preferred embodiment of the invention, a temperature chamberencloses the friction surface and the sample whereby temperature can becontrolled to simulate different temperature conditions. Additionally oralternatively, the machine includes a water source and a pump whichtransfers water from the source onto the friction surface whereby themeasurement device can obtain measurements indicative of the frictionalforce in wet conditions.

In a preferred embodiment of the invention, a processor controls themotion device, controls the variable weight loading device, controls thetemperature chamber, and/or records the measurements obtained by theforce measurement device. The processor further analyzes themeasurements and compiles the results in the form of tables and/orgraphs and an output device is provided to allow a readout of thecompiled results. Preferably, the processor provides a library ofpre-defined tests and a selection device for selecting a particulartest.

The present invention accordingly provides a method of predicting tiretraction characteristics of a tread component. The method comprises thesteps of preparing a sample of the tread component; attaching the sampleto the sample holder, setting the variable weight device to apply acertain load to the sample, and using the force measurement device forobtaining a measurement indicative of the frictional force resistingsuch relative movement between the sample holder and the frictionsurface effected by the motion device. During this method, the frictionsurface may be replaced with another different friction surface topredict tire traction characteristics with respect to different roadsurfaces. Also, water may be transferred onto the friction surface togather measurements indicative of the frictional force in wetconditions. The temperature surrounding the friction surface and thesample may be controlled to simulate different temperature conditions,such as winter conditions and summer conditions.

The foregoing and other features of the invention are hereinafter fullydescribed and particularly pointed out in the claims, the followingdescription and the annexed drawings setting forth in detail one or moreillustrative embodiments of the invention, such being indicative,however, of but one or a few of the various ways in which the principlesof the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view of a portable frictiontesting machine according to the invention.

FIG. 2 is a schematic plan view of the portable friction testing machineof FIG. 1.

FIG. 3 is a partial sectional view taken along the line 3—3 of FIG. 1showing details of a representative friction block used in the machine.

FIG. 4 is a schematic plan view of another embodiment of portablefriction testing machine according to the invention.

FIG. 5 is a schematic side elevational view of the portable frictiontesting machine of FIG. 3.

FIG. 6 is a schematic side elevational view of a further embodiment ofportable friction testing machine according to the invention.

FIG. 7 is a schematic plan view of the portable friction testing machineof FIG. 6.

FIG. 8 is a schematic side view of another embodiment of a portablefriction testing machine according to the invention.

FIG. 9 is a schematic side plan view of another embodiment of a portablefriction testing machine according to the invention.

FIG. 10 is a schematic top view of the portable friction testing machineof FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the drawings and initially to the embodimentof the invention shown in FIGS. 1-3, a portable friction testing machineaccording to the invention is schematically shown at 20. The machine 20comprises a base 22 in the form of a plate. In the embodimentillustrated in FIGS. 1-3, the base 22 has a length several times longerthan its width and is supported atop a cart 24 that may be equipped withwheels, such as casters 25, for easy transport from one place toanother. As diagrammatically shown, the cart 24 also carries, fortransport with the other components of the machine 22, electroniccomponents 26 including a processor 28. The processor 28 can be aconventional microcomputer suitably programmed to carry out the variouscontrol and processing functions of the machine. As will be appreciated,the machine may be otherwise supported and configured, preferably foreasy portability. For example, the machine alternatively may besupported on a table top and the base may be equipped with handles forconvenient lifting of machine for transport from one table to another.

As best seen in FIG. 3, the base 22 includes a recess 36 for receiving afriction element in the form of a block 38. The friction block 38includes a holder 40 for containing a friction material 42 that has anexposed top or friction surface 44. The holder 40 and recess 36 arecorresponding sized and configured to prevent any significant horizontalshifting of the holder relative to the platform while permitting easyremoval of the friction block from the base as by lifting the frictionblock out of the recess in the base. This enables easy and quickinterchanging of different friction blocks having different frictionsurfaces, as may be desired for testing a particular sample on differentfriction surfaces. The friction surfaces can be composed of differentmaterials and/or textures. As will be appreciated, the friction blockmay be entirely formed of a single material in which case the holder andfriction material are integral with one another. Preferably, thefriction surface 44 is planar and flush with the top surface 46 of thebase 22 and extends parallel to the longitudinal axis of the base 22about approximately half the length of the base. Like the base, thefriction surface has a length several times longer than its width.

The friction surface 44 may be any one of a variety of differentsurfaces. The surface may be composed of asphalt, concrete, snow, etc.and the surface may have different textures, for example rough, smooth,grooved, etc. If desired, the friction surface may have differentcharacteristics along the length thereof, as is desirable for measuringfriction characteristics of the sample as it transitions from onesurface to another. For example, the first half of the friction surfacemay be composed of asphalt and the second half may be composed ofconcrete. The friction block can also be transparent to enable viewingof the sample as it moves across the friction surface. For example, thefriction block can be made of glass or plexiglass having a frictionsurface formed by texturing, by a wire mesh on top of the glass orplexiglass substrate, etc. A camera, particularly a high speed camera,can be mounted beneath the friction block to view and record the dynamicaction of the sample as it moves across the friction surface.

The base 22 has mounted thereon a vertical support plate 50 which, inthe illustrated embodiment, extends approximately half the length of thebase 22. The vertical support plate 50 has at opposite ends thereofin-turned mounting arms 52 and 54. Connected to and extending betweenthe mounting arms are a pair of guides 56 in the form of rods on which acarriage 58 is mounted and guided for movement along the length of thefriction surface. The carriage preferably is equipped with suitablebearings for relatively friction-free sliding on the guide rods.

The carriage 58 carries a sample holder 62. The sample holder 62includes a post 64 that is guided by suitable bearings in the carriage58 for vertical movement perpendicular to the friction surface. Attachedto the lower end of the post 64 is a sample mount 66 to which a sample Scan be removably attached by suitable means.

Attached to the upper end of the post 64 is a weight platform 68 onwhich weights W can be removably stacked to selectively vary the loadapplied to the sample. Preferably, the weights are donut shape orotherwise have a center hole for slipping over the upper end of the postor other projection extending upwardly from the weight platform 68 tohold the weights W on the platform as the carriage is moved. However,other means may be employed to hold the weights in place while stillpermitting easy assembly or removal of the weights on or from theplatform. The amount of weight set on the platform 68 determines theamount of normal force holding the sample against the friction surface44 as the sample is moved over the friction surface. Preferably, alifting device 70 is provided to lift the sample S off of the frictionsurface for changing of the sample S and/or friction surface 44 duringtesting procedures.

Movement of the sample S over the friction surface 44 is effected by amotion device 76. In the illustrated embodiment, the motion deviceincludes a motor 78 which rotates a lead screw 80 for linearly moving adrive carriage 82 along the longitudinal axis of the base 22. The drivecarriage 82 is connected to the specimen holder 66 or carriage 58 by aconnecting member 84 for moving the specimen holder along with the drivecarriage. In the illustrated embodiment, the connecting member 84 is arod connected by a ball joint 86 at one end to the drive carriage and bya ball joint 88 at its opposite end to the specimen holder 66. The balljoints 86 and 88 accommodate slight misalignments between the specimenholder/carriage path and the drive carriage path. The drive motor 78 canbe operated in either direction for moving the specimen holder over thefriction surface in opposite directions. Moreover, the drive motor canbe operated at different speeds to provide a large range of slidingvelocities. For example, when measuring the coefficient of friction of atread compound, the motor can be operated at sliding velocitiescomparable to what a tire experiences in service. For example,translation speeds in the range of zero to about 4.0 inches/second canbe obtained. As will be appreciated, other connecting members can beused, such as a wire to pull the specimen holder/carriage in onedirection over the friction surface.

The drive carriage 82 has secured thereto a force measurement device 92,such as a load cell, which measures the horizontal force at thefootprint of the sample. It is the load cell 92 to which the connectingrod 88 is connected by the ball joint 86. As the drive carriage movesthe sample holder 66 over the friction surface 44, the load cell willoutput a signal indicative of the frictional force resisting movement ofthe sample across the friction surface. The output of the load cell isconnected to a data acquisition system 94. More particularly, the outputof the load cell 92 is connected by a shunt calibrator 96 to a chargeamplifier 98 which in turn is connected to the processor 28. As thoseskilled in the art will appreciate, the shunt calibrator is provided foreasy calibration of the system. The processor 28 processes the output ofthe sensor to provide a measurement of the friction characteristics,e.g., coefficient of friction, of the sample S being tested for theselected friction surface.

In the case of a tread element, for example, it may be desirable tomeasure the coefficient of friction in different directions. To thisend, the sample holder 66 can be rotated about its vertical axis (normalto the friction surface) in the carriage 58 and then fixed by suitablemeans at a selected angle of rotation.

For some tests, It is desirable to give the sample two independenttranslations in two orthogonal directions, a translation and rotation,or both. The linear movement of the sample S across the friction surfaceeffected by the motor 78 constitutes translation in one orthogonaldirection. To obtain translation in a second orthogonal direction, thereis provided a transverse drive mechanism 100 for moving the sampleholder transversely back and forth relative to the linear movementdirection of the carriage. To obtain rotation about an axisperpendicular to the friction surface, a rotation mechanism 102 isprovided for rotating the sample holder during movement of the samplelinearly across the friction surface. Preferably, the transverse drivemechanism 100 and rotation mechanism 102 are housed in the carriage 58,as diagrammatically depicted in FIG. 1, and operate on the post 64 foreffecting transverse and rotational movement of the sample holder 66.For example, the rotation mechanism 102 can be a motor and suitablegearing and/or other drive components for rotating the sample holder ina controlled manner. For some tests, it may be desirable to reciprocallyrotate the sample back and forth as it is moved along the frictionsurface. Similarly, the transverse drive mechanism 100 can be a motorand suitable gearing and/or other drive components for moving the sampleholder transversely to the direction of movement of the carriage alongthe length of the friction surface. If the sample holder 66 isundergoing rotational or transverse movement during testing, thenpreferably the connecting member 84 is connected to the carriage 58, andthus indirectly to the sample holder instead of directly to the sampleholder 66.

Tests can also be conducted under wet conditions. To this end, water canbe applied to the friction surface 44 by a tube 108 connected to areservoir 110 or other source of water. In the illustrated embodiment, apump 112 is used to pump the water onto the friction surface 44 at adesired rate or when needed. A suitable drain 114 (FIG. 3) is preferablyprovided, for example at the bottom of the recess 36 in the base 22, forremoving water from the test area and, if desired, recycling the waterback to the reservoir 110 as shown in FIG. 1. For wet tests, thefriction block preferably is provided with suitable drain passages, asalong the edges thereof for channeling the water to the drain, asopposed to the water flowing over the top surface of the base. However,it will be appreciated that the base alternatively or additionally maybe configured for collection of the water being applied to the frictionsurface. Also, a distribution member, such as a manifold with multipleoutlets spaced along the length of the friction surface, can be employedfor more even distribution of water over the friction surface.

Referring now to FIGS. 4 and 5, another embodiment of portable frictiontesting machine according to the invention is schematically shown at120. The machine 120 comprises a base 122 supported on or in the form ofa cart that may be equipped with wheels, such as casters, for easytransport from one place to another. Like in the above described testingmachine 20, the cart preferably carries, for transport with the othercomponents of the machine 122, electronic components including aprocessor 128. As before, the processor 128 can be a conventionalmicrocomputer suitably programmed to carry out the various control andprocessing functions of the machine.

The base 122 has mounted thereon a table 132 that rotates about avertical axis. The table is driven by a motion device 134 including, forexample, a motor and suitable controls for controlling the speed of themotor. The outer peripheral annular edge portion of the table is coveredby a friction surface preferably provided by a removable annularfriction element 138. The removable friction element 138 is in the formof an annular disc-like holder for containing a friction material thathas an exposed annular top or friction surface 144. The annular frictionelement 138 is concentric with the rotation axis of the rotating table132 and is suitably secured to the rotating table by suitable means forrotation therewith. Preferably, the friction element is removablysecured for permitting easy and quick interchanging of differentfriction elements having different friction surfaces, as may be desiredfor testing a particular sample on different friction surfaces. Asbefore, the surfaces can be composed of different materials and/ortextures, such as those above mentioned.

The base 122 has mounted thereon a carriage 158 located above thefriction surface 144 on the rotating table 132. The carriage 158 hasmounted therein a sample holder 162 which includes a post 164 that isguided by suitable bearings in the carriage 158 for vertical movementperpendicular to the friction surface 144. Attached to the lower end ofthe post 164 is a sample mount 166 to which a sample S can be removablyattached by suitable means. Attached to the upper end of the post is aweight platform 168 on which weights W can be removably stacked toselectively vary the load applied to the sample, as in the mannerdescribed above in connection with the testing machine 20. The amount ofweight set on the platform 168 determines the amount of normal forceholding the sample against the friction surface as the sample is movedover the friction surface.

As will be appreciated, movement of the sample S relative to thefriction surface 144 is effected by rotating the table 132. The drivemotor 136 can be operated in either direction for moving the frictionsurface relative to the sample in opposite directions. Moreover, thedrive motor can be operated at different speeds to provide a large rangeof sliding velocities, such as the above mentioned range. The rotatingtable preferably is of a sufficiently large diameter that the sample inessence is moving linearly relative to the friction surface, thisessentially being equivalent to the linear translating movement of thesample in the testing machine 20.

The sample mount 166 includes a force measurement device 192, such as aload cell, which measures the horizontal force at the footprint of thesample S parallel to the movement direction of the sample relative tothe friction surface 144. As the motor 136 moves the friction surface144 underneath the sample holder 158, the load cell 192 will output asignal indicative of the frictional force resisting movement of thesample across the friction surface. The output of the load cell isconnected to a data acquisition system as above described.

As above indicated, it may be desirable to measure the coefficient offriction in different directions. To this end, the sample holder 166 canbe rotated about its vertical axis (normal to the friction surface) inthe carriage 158 and then fixed by suitable means at a selected angle ofrotation.

Again, for some tests, It is desirable to give the sample twoindependent translations in two orthogonal directions, a translation androtation, or both. The movement of the rotating friction surface 144beneath the sample S essentially constitutes translation in oneorthogonal direction. To obtain translation in a second orthogonaldirection, a transverse drive mechanism 200 for moving the sample holdertransversely back and forth relative to the linear movement direction ofthe carriage. To obtain rotation about an axis perpendicular to thefriction surface, a rotation mechanism 202 is provided for rotating thesample holder relative to the carriage while the friction surface ismoving beneath the sample. Preferably, the transverse drive mechanism200 and rotation mechanism 202 are housed in the carriage 158 asdiagrammatically depicted in FIG. 5.

FIGS. 6 and 7 show a further embodiment of portable friction testingmachine according to the invention. The machine 320 is substantially thesame as the above described machine 20, except that it modified asdiscussed below to provide for in situ testing of a friction surface,such a floor surface or roadway surface. Thus, the electronic componentsof the machine 320 are the same as above described in connection withmachine 20, although the processor is not shown in FIGS. 6 and 7.

Like the machine 20, the machine 320 comprises a base 322 in the form ofa plate. The base 322 is adapted to rest atop the friction surface 344against which a sample is to be tested. If desired, the base may becarried by a cart (not shown) that may be equipped with wheels, such ascasters, for easy transport from one place to another. The cart may beequipped with a mechanism for lowering the base, when testing isdesired, to a position adjacent a friction surface on which the cart issupported. As will be appreciated, the machine may be otherwisesupported and configured, preferably for easy portability. For example,the machine alternatively may be equipped with handles for convenientlifting of machine for transport from one place to another.

The base 322 includes an aperture and more particularly an elongatedthrough opening 336 for permitting access to the underlying frictionsurface by a sample S in a sample holder 362. The sample holder 362includes a post 364 that is guided by suitable bearings in a carriage358 for vertical movement perpendicular to the friction surface.Attached to the lower end of the post 364 is a sample mount 366 to whichthe sample S can be removably attached by suitable means. As seen inFIG. 6, the sample holder (including the sample) extends through theopening 336 in the base for engagement with the friction surfaceunderlying the base.

The upper end of the post 364 is provided with a weight platform 368 onwhich weights W can be removably stacked to selectively vary the loadapplied to the sample. As in the case of the machine embodiment of FIGS.1 and 2, the amount of weight set on the platform 368 determines theamount of normal force holding the sample against the friction surfaceas the sample is moved over the friction surface. Likewise, the base 322has mounted thereon a vertical support plate 350. Connected to andextending between mounting arms on the support plate are a pair ofguides 356 in the form of rods on which a carriage 358 is mounted andguided for movement along the length of the elongated opening 336 in thebase.

Movement of the sample S over the friction surface 344 is effected by amotion device 376 including, for example, a motor 378 which rotates alead screw 380 for linearly moving a drive carriage 382 along thelongitudinal axis of the base 322. The drive carriage 382 is connectedto the specimen holder 366 or carriage 358 by a connecting member 384for moving the specimen holder along with the drive carriage. The drivemotor 378 can be operated in either direction for moving the specimenholder over the friction surface in opposite directions. Moreover, thedrive motor can be operated at different speeds to provide a large rangeof sliding velocities.

The drive carriage 382 has secured thereto a force measurement device392, such as a load cell, which measures the horizontal force at thefootprint of the sample. As the drive carriage moves the sample holder366 over the friction surface 344, the load cell will output a signalindicative of the frictional force resisting movement of the sampleacross the friction surface. The output of the load cell is connected toa data acquisition system 394 as above described in connection with themachine embodiment 20 of FIGS. 1 and 2. The output of the sensorprovides a measurement of the friction characteristics, e.g.,coefficient of friction, of the sample S being tested for the selectedfriction surface. As before described, the sample holder 366 can berotated about its vertical axis (normal to the friction surface) in thecarriage 358 and then fixed by suitable means at a selected angle ofrotation. Additionally or alternatively, the sample may be given twoindependent translations in two orthogonal directions, a translation androtation, or both. The linear movement of the sample S across thefriction surface effected by the motor 378 constitutes translation inone orthogonal direction. To obtain translation in a second orthogonaldirection, there is provided a transverse drive mechanism 400 for movingthe sample holder transversely back and forth relative to the linearmovement direction of the carriage. To obtain rotation about an axisperpendicular to the friction surface, a rotation mechanism 402 isprovided for rotating the sample holder during movement of the samplelinearly across the friction surface. Preferably, the transverse drivemechanism 400 and rotation mechanism 402 are housed in the carriage 358,as diagrammatically depicted in FIG. 6, and operate on the post 364 foreffecting transverse and rotational movement of the sample holder 366.

By way of example, the aforesaid testing machines can be used predicttire traction characteristics of a tread component. First, a sample ofthe tread component is prepared and attached to the sample holder ineither one of the above described friction test machines. Then, one ormore weights can be placed on the sample holder for loading the sampleholder such that a selected load is applied to the sample normal to thefriction surface. The test machine is then operated to slide the sampleover the friction surface while data is collected by the processor toprovide measurements indicative of the frictional force resistingrelative movement between the sample holder and the friction surfaceeffected by the motion device. The resistance force can be used tocalculate the coefficient of friction of the sample relative to thefriction surface. As above discussed, the friction surface, in the firsttwo embodiments described above, is replaceable with different frictionsurfaces for predicting tire traction characteristics with respect todifferent road surfaces. In the third embodiment above described, thetesting machine can be positioned on different surfaces, such asdifferent roadway surfaces. As will be appreciated, the testing machinecan be used to determine the frictional characteristics of a sample, forexample a section of an actual tire tread, in relation to a frictionsurface, in situ, for example a roadway surface. Referring now to FIG.8, a schematic plan view of another friction testing machine 520according to the present invention is shown. The machine 520 is similarto the machine 20 in that it includes a base 522 supported a top a cart524 (with castors 525). However, the machine 520 may be modified toaccommodate in situ testing by, for example, adopting a supportstructure similar to that of machine 320. The machine 520 includes aprocessor 528 which, as with the processor 28 of machine 20 discussedabove, can be a conventional microcomputer suitably programmed to carryout the various control functions of the machine.

The friction testing machine 520 additionally comprises a frictionsurface 544, a carriage 558, and a motion device 576 which causesrelative movement between the carriage 558 and the friction surface 544.A force measurement device 92, such as a load cell, obtains measurementsindicative of the relevant friction force during operation of themachine 520. A sample holder 566 is mounted to the carriage 558 to holda sample S in frictional engagement with the friction surface 544 duringmovement of the carriage 558. Although not specifically shown in thedrawing, the friction surface 544 is preferably formed in the samemanner as the friction surface 44 of the machine 20. Specifically, aselected one of a plurality of friction elements is removably supportedrelative to the carriage 558 whereby friction elements can beinterchanged to provide different friction testing surfaces.

The motion device 576 is similar to the motion device 76 in that itincludes a motor 578 which rotates a lead screw 580 for linearly movingthe carriage 558 and thus the sample S held by the sample holder 562. Aswith the motion device 78, the drive motor 578 can be operated in eitherdirection for moving the specimen holder over the friction surface inopposite directions so that the force measurement device can obtainmeasurements indicative of the frictional force in forward and reversedirections. The machine 520 may additionally include a transversemechanism 100 and/or a rotation mechanism 102 for effecting relativemovement between the sample holder and friction surface in a directiondifferent from the forward and reverse directions.

The machine 520 includes a variable weight loading device 568 on thecarriage 558 loads the sample holder 562 so that a selected load can beapplied to the sample in a direction normal to the friction surface 544.The preferred and illustrated variable weight loading 568 is a fluidcylinder that is moved to different positions to selectively vary theload applied to the sample. The processor 528 preferably controls thevariable weight loading device whereby a fluid cylinder, as opposed to aweight platform is preferred.

The machine 520 preferably includes a temperature control chamber 700enclosing at least the friction surface 544 and the sample S wherebytemperature can be controlled to simulate different temperatureconditions. For example, the chamber can be cooled to freezing orbelow-freezing temperatures to simulate winter temperature conditionsand/or heated to simulate summer temperature conditions. It may be notedthat although in the illustrated embodiment, the chamber 700 enclosesthe weight platform 568, it may be instead provided with a top slot toaccommodate an extension of the platform and its supporting post abovethe chamber. The machine 620 may additionally or alternatively includecomponents (such as tube 508, reservoir 510, and pump 512) to transferwater onto the friction surface 544 to simulate wet driving conditions.

The processor 528 preferably controls the motion device 576 and theweight loading device 568 to apply the desired test conditions duringoperation. The processor 528 may also control the temperature chamber700 and/or the wetting components. In any event, the processor 528preferably analyzes the measurements and compiles the results in theforms of tables and/or graphs and provides a read-out of these compiledresults. Preferably, the processor 528 provides a library of pre-definedtests whereby a particular test may be selected.

Referring now to FIGS. 9 and 10, schematic plan and top views of anotherfriction testing machine 620 according to the present invention isshown. The machine 620 may include a base 622 that may be supported on amobile cart. Alternatively, the machine 620 may be modified toaccommodate in situ testing by, for example, adopting a supportstructure similar to that of machine 320. The machine 620 also includesa processor 628 which, as with the processors 28 and 528 discussedabove, can be a conventional microcomputer suitably programmed to carryout the various control functions of the machine.

The friction testing machine 620 has a friction surface 644 which,although not specifically shown in the drawing, is preferably formed inthe same manner as the friction surface 44 of the machine 20.Specifically, the base 622 includes a recess for removable receipt of afriction element in the form of a block. In this manner, differentfriction elements can be easily and quickly interchanged as may bedesired for testing a particular sample on different friction testingsurfaces.

The base 622 includes a pair of guides 656 in the form of horizontalrods on which a drive carriage 658 is mounted and guided for movement. Asample carriage 660 is attached to the drive carriage 658 via linearbearings and the sample carriage 660 carries a sample holder 662. Thesample holder 662 includes a post 664 that is guided by suitablebearings in the carriage 660 for vertical movement perpendicular to thefriction surface 644. Attached to the lower end of the post 664 is asample mount 666 to which a sample S is removably attached by suitablemeans. Attached to the upper end of the post 664 is a weight platform668, similar to the weight platform 68. Although not specifically shownin the drawings, the machine 620 may also include a lifting device, suchas the lifting device 70 of machine 20, to facilitate changing of thesample and/or friction surface during testing procedures. Alternatively,the fluid cylinder, such as the fluid cylinder 568 of machine 520, maybe used instead and controlled by the processor 628.

Movement of the sample S over the friction surface 644 is effected by amotion device 676. The motion device 676 is especially designed to movethe drive carriage 658 in a reciprocating manner repeatedly during thecollection of friction data. This reciprocating approach reduces thelength of the friction surface 644 necessary for the measurements and/ordata from these repeated measurements can be averaged for increasedaccuracy.

The motion device 676 includes a motor 678 which rotates a lead screw680 for linearly moving the drive carriage 658 along the guide rods 656.The drive motor 678 can be operated in either direction and can beoperated at different speeds to provide a large range of slidingvelocities. For example, when measuring the coefficient of friction of atread compound, the motor can be operated at sliding velocitiescomparable to what a tire experiences in service. For example,translation speeds in the range of zero to about 4.0 inches/second canbe obtained. The motor 678 is preferably controlled by the processor 628via a motor controller 690.

The motor 678 is positioned so that the lead screw 680 is positionedparallel with and in between the guide rods 656. The sample carriage 660has secured thereto a force measurement device 692, such as a load cell,to measures the horizontal force at the footprint of the sample.Specifically, as the drive carriage 658 is moved by the motion device676 in a first direction, the sample S is moved over the frictionsurface 644. The resistance frictional forces encountered by the sampleS will restrict the movement of the sample carriage 660 in the firstdirection whereby the force measurement device 692 must “pull” or “push”the carriage 660 to compensate for this restriction. In this manner, thedevice, or more particularly the load cell 692, outputs a signalindicative of the frictional force resisting movement of the sampleacross the friction surface 644. The output of the load cell 692 isconnected to the processor 628 which processes the output of the sensorto provide a measurement of the friction characteristics, e.g.,coefficient of friction, of the sample S being tested for the selectedfriction surface.

The processor 628 preferably controls the motion device 676 duringoperation of the machine 520. In this matter, the stepper motor 676 canbe programmed to pull the specimen S over the friction surface 644 atany desired velocity as a function of time. For example, the specimen Scould be pulled in such a way that its velocity first increases withtime and then decreases with time. Additionally or alternatively, thestepper motor 676 can be programmed to move the specimen in areciprocating manner repeatedly (in some applications, up to severalthousand times) and collect the friction data. The processor 628preferably analyzes the measurements and compiles the results in theforms of tables and/or graphs and provides a read-out of these compiledresults. Preferably, the processor 628 provides a library of pre-definedtests whereby a particular test may be selected. Position sensors 696may be provided to provide input to the controller 628 as to the actualposition of the drive carriage 658 during the testing procedures.

The machine 620 may additionally include a transverse mechanism 100and/or a rotation mechanism 102 for effecting relative movement betweenthe sample holder and friction surface in a direction different from theforward and reverse directions. Additionally or alternatively, themachine 620 may includes a temperature control chamber (such as thechamber 700, discussed above) enclosing at least the friction surface644 and the sample S whereby temperature can be controlled to simulatedifferent temperature conditions. Another option is to includecomponents (such as the tube 108, the reservoir 110, the pump 112, andthe drain 114 of machine 20) to transfer water onto the friction surface644 to simulate wet driving conditions. The processor 628 may alsocontrol the transverse/rotational mechanisms, the temperature chamberand/or the wetting components.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described integers (components, assemblies,devices, compositions, etc.), the terms (including a reference to a“means”) used to describe such integers are intended to correspond,unless otherwise indicated, to any integer which performs the specifiedfunction of the described integer (i.e., that is functionallyequivalent), even though not structurally equivalent to the disclosedstructure which performs the function in the herein illustratedexemplary embodiment or embodiments of the invention. In addition, whilea particular feature of the invention may have been described above withrespect to only one of several illustrated embodiments, such feature maybe combined with one or more other features of the other embodiments, asmay be desired and advantageous for any given or particular application.

What is claimed is:
 1. A friction test system comprising: a carriage; afriction surface; a motion device which causes relative movement betweenthe carriage and the friction surface in at least a first direction; asample holder mounted to,the carriage which holds a sample in frictionalengagement with the friction surface during the relative movementbetween the carriage and the friction surface; a variable weight loadingdevice carried by the carriage which loads the sample holder so that aselected load can be applied to the sample in a direction normal to thefriction surface; a force measurement device which obtains a measurementindicative of the frictional force resisting movement of the sample asit is moved in the first direction; and a processor which controls themotion device, controls the variable weight loading device and/orrecords the measurements obtained by the force measurement device;wherein the motion device causes relative movement between the carriageand the friction surface in a second direction opposite to the firstdirection so that the force measurement device can obtain measurementsindicative of the frictional force in forward and reverse directions. 2.A friction test system as set forth in claim 1, wherein the processorfurther analyzes the measurements and compiles the results in the formsof tables and/or graphs.
 3. A friction test system as set forth in claim1, wherein the processor provides a library of pre-defined tests and aselection device for selecting a particular test.
 4. A friction testsystem comprising: a carriage; a friction surface; a motion device whichcauses relative movement between the carriage and the friction surfacein at least a first direction; a sample holder mounted to the carriagewhich holds a sample in frictional engagement with the friction surfaceduring the relative movement between the carriage and the frictionsurface; a variable weight loading device carried by the carriage whichloads the sample holder so that a selected load can be applied to thesample in a direction normal to the friction surface; a forcemeasurement device which obtains a measurement indicative of thefrictional force resisting movement of the sample as it is moved in thefirst direction; and a processor which controls the motion device,controls the variable weight loading device and/or records themeasurements obtained by the force measurement device; furthercomprising a second motion device for effecting relative movementbetween said sample holder and friction surface in another directiondifferent than said first direction.
 5. A friction test system as setforth in claim 4, wherein the other direction is rotation about an axisperpendicular to the friction surface.
 6. A friction test machine as setforth in claim 4, wherein the other direction is transverse to said onedirection.
 7. A friction test system comprising: a drive carriage; afriction surface; a motion device which causes relative movement betweenthe drive carriage and the friction surface in a first direction and asecond opposite direction; a sample holder mounted to the carriage whichholds a sample in frictional engagement with the friction surface duringthe relative movement between the drive carriage and the frictionsurface; a variable weight loading device which loads the sample holderso that a selected load can be applied to the sample in a directionnormal to the friction surface; a force measurement device which obtainsa measurement indicative of the frictional force resisting movement ofthe sample as it is moved in the first and second directions therebyobtaining measurements indicative of the frictional force in the forwardand reverse directions.
 8. A friction test system as set forth in claim7, wherein a sample carriage is attached to the drive carriage vialinear bearings and wherein the sample holder is mounted to the samplecarriage.
 9. A friction test system as set forth in claim 8, wherein thebase includes a pair of guide rods on which the drive carriage ismounted and guided for movement and wherein the motion device includes amotor which rotates a lead screw for linearly moving the drive carriagealong the guide rods.
 10. A friction test system as set forth in claim9, wherein the lead screw is positioned parallel with and between theguide rods.
 11. A friction test system as set forth in claim 10, whereinthe force measure device is secured to the sample carriage.
 12. Afriction test system as set forth in claim 11, further comprising aprocessor, wherein the force measurement device is a load cell thatprovides output signals indicative of frictional forces resistingmovement of the sample; wherein the output signals of the load cell areprovided to the processor which processes these signals to provide ameasurement of the friction characteristics of the sample S being testedon the friction surface.
 13. A friction test system as set forth inclaim 12 wherein the variable weight loading device is carried by thesample carrier.
 14. A friction test system as set forth in claim 13wherein the sample holder comprises a post that is guided by suitablebearings in the sample carriage for vertical movement perpendicular tothe friction surface and a sample mount attached to the lower end of thepost to which the sample S is removably attached.
 15. A friction testsystem as set forth in claim 14 wherein the variable weight loadingdevice is attached to the upper end of the post.
 16. A friction testsystem as set forth in claim 7 further comprising a lifting device tolift the sample holder away from the friction surface between testingprocedures.
 17. A method of measuring the coefficient of frictionbetween a sample and a friction surface with the friction test system ofclaim 7, said method comprising the steps of: mounting a sample in thesample holder; and activating the motion device to cause relativemovement between the drive carriage and the friction surface in thefirst and second direction.
 18. A method as set forth in claim 17,wherein the activating step includes programming the motion device tomove the drive carriage repeatedly in a reciprocating manner to collectfriction data.
 19. A method as set forth in claim 18, further comprisingthe step of averaging together the measurements obtained by the forcemeasurement device during the reciprocating movement.
 20. A method asset forth in claim 17, wherein the motion device is operated atdifferent speeds to provide a range of sliding velocities.
 21. A methodof measuring the coefficient of friction between a sample and a frictionsurface with the friction test system of claim 1, said method comprisingthe steps of: mounting a sample in the sample holder; and activating themotion device to cause relative movement between the carriage and themotion device in the first direction; and recording measurementsobtained by the force measurement device; wherein said activating andsaid recording steps are performed by the processor.